Expandable styrene resin particles, expandable beads, and foamed article

ABSTRACT

A process of producing expandable styrene resin particles, wherein in suspension polymerization of styrene monomers, the concentration of oxygen in a reaction vessel is kept low at least in a late stage of the polymerization, and the resulting styrene resin particles are impregnated with an expanding agent before or after completion of the polymerization. Preferably, at a polymerization rate of 60% or higher, the concentration of oxygen in the reaction vessel is kept at 7 volt % or lower. When additional styrene monomers are added during the polymerization, they are added and adsorbed to styrene resin particles in the course of polymerization while the concentration of oxygen is kept low. By this method, the particles whose internal portion has a low molecular weight and surface portion has a high molecular weight are obtained. A foamed article produced from the particles has high strength and a good appearance.

This application is a continuation application of U.S. application Ser.No. 10/430,409, filed May 7, 2003 now U.S. Pat. No. 6,797,733 thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

The present invention relates to expandable styrene resin particles anda production process thereof, expandable styrene beads, and a foamedarticle.

(ii) Description of the Related Art

An expandable styrene resin is used for a number of food containers,packing materials, cushioning materials and others by taking advantageof its excellent heat insulation properties, economy and sanitation. Afoamed article thereof is produced by heating particles of theexpandable styrene resin by steam or the like to be pre-foamed to adesired bulk density, subjecting the particles to an aging step, andfilling the resulting particles into a mold to be foamed under heatingagain. The expandable styrene resin particles are required to haveexcellent expandability in the pre-expanding step or the expand-moldingstep in the mold under heating and to have high strength or an excellentappearance when molded into an article.

Heretofore, to enhance the strength of the foamed article, some methodshas been investigated, for example, increasing of the density of amolded article, or increasing of the thickness of the molded articleitself. However, any of these methods can be hardly considered to beeconomical since the weight of the molded article increases and alsohave problems from an environmental standpoint. As for the expandablestyrene resin particles, some methods has been investigated, forexample, increasing the molecular weight, or controlling or reducing akind or an amount of plasticizer for plasticizing the resin particles.

On the other hand, to improve the expandability of the expandablestyrene resin particles, a method of decreasing the molecular weight, amethod of plasticizing the resin particles, a method of changing acomposition ratio of expanding agents and other methods have beeninvestigated.

Such methods of enhancing the strength of the foamed article and suchmethods of improving the expandability are generally contradictory toeach other, and hence, it has been difficult to simultaneously satisfyboth the methods.

For solving these problems, resin particles, whose outermost layerportion and central portion have a low molecular weight and middleportion has a high molecular weight, are proposed in Japanese PatentLaid-Open No. 295756/1996.

However, the above particles have a problem that since the molecularweight of the outermost layer portion is low like the central portion,thermal fusion is promoted excessively at the time ofheat-expand-molding, thereby degrading a surface finish of a moldedarticle.

Further, in Japanese Patent Laid-Open No. 188454/1995, resin particleswhose surface layer portion has a higher molecular weight than that ofthe whole particle are disclosed.

It is described in this publication that when the surface layer portionhas an excessively high molecular weight, the expandability deterioratesand the appearance of the resulting molded article is degraded with alower strength. This is assumed to be because the molecular weight ofthe central portion of the particles disclosed therein also becomes highas the molecular weight of the surface layer portion becomes high. Thatis, the above resin particles have a problem that the molecular weightof the surface layers cannot be sufficiently increased.

Further, heretofore, to obtain the foamed articles having a goodappearance, it was essential to eliminate gaps among beads completelywhen styrene beads filled in a mold are foamed under heating. However,it was difficult to eliminate all gaps among the beads. Under thecircumstances, to eliminate as many voids as possible, the properties ofexpandable styrene resin particles themselves and molding techniquesincluding the functions of molding machines and the like have beenimproved.

For example, to improve the properties of the expandable styrene resinparticles themselves, efforts were made to control the type and amountof plasticizer for plasticizing the resin particles, or to render themolecular weight lower. However, these methods lead to a reduction inthe heat resistance of the resin particles and have a problem that thesurface of a molded article is molten by heat-expanding at the time ofmolding, thereby increasing voids.

Further, to improve molding techniques including the functions ofmolding machines and the like, the following have been studied. As forthe molding machines, a control method in a heating step has beenstudied, and a method of using steam more efficiently was employed. Asfor molds, a method of using steam more effectively by, for instance,increasing the number of slits has been studied. However, these methodshave problems that additional costs including costs for improving themachines and the molds are so large and it is difficult to improve allof these at once.

SUMMARY OF THE INVENTION

An object of the present invention is to provide expandable styreneresin particles capable of forming a molded article having high strengthand having excellent expandability, expandable beads, and a foamedarticle.

Further, another object of the present invention is to provideexpandable styrene resin particles capable of forming a molded articlehaving a good appearance and having excellent expandability, expandablebeads, and a foamed article.

A process for producing expandable styrene resin particles according tothe present invention is characterized in that in suspensionpolymerization of styrene monomers, the polymerization reaction isallowed to proceed, keeping the concentration of oxygen in a reactionvessel low at least in the late stage of the polymerization.

The process can prevent decreases in molecular weight of a surfaceportion.

In the process of the present invention, the polymerization may becarried out with seeds.

Further, the addition of styrene monomers in the late stage ofpolymerization leads to higher molecular weight of a surface portion,improving the strength of a molded article. The polymerization may becarried out without the addition of monomers.

When adding styrene monomers in the late stage of the polymerization,they are added while the concentration of oxygen is kept low.

For example, according to one embodiment of the process of the presentinvention, in a suspension polymerization of styrene monomers, when arate of polymerization is at least 60%, additional styrene monomers areadded and adsorbed to styrene resin particles in the course ofpolymerization while the concentration of oxygen in a reaction vessel iskept at 7 vol % or lower so as to allow a polymerization reaction toproceed, and the particles are impregnated with an expanding agentbefore or after completion of the polymerization reaction.

By this method, the following expandable styrene resin particles can beobtained.

1. An expandable styrene resin particle, wherein the weight averagemolecular weight of a surface portion from the surface of the particleto a depth of ⅕ of its radius toward the center is higher than that of acentral portion from the center to a distance of ⅕ of the radius towardthe surface, and

a chart of gel permeation chromatography of the surface portion has twocrests or a shoulder.

2. The particle of the paragraph 1, wherein the weight average molecularweight of the central portion is 200,000 to 300,000,

the weight average molecular weight of the surface portion is 300,000 to450,000, and

the weight average molecular weight of the surface portion is at least1.2 times as large as that of the central portion.

3. An expandable styrene resin particle, wherein the weight averagemolecular weight of a resin portion forming 30 to 60 wt % from thecenter toward the surface of the particle is 200,000 to 300,000,

the weight average molecular weight of a resin portion forming 60 to 30wt % from the surface toward the center of the particle is 300,000 to450,000, and

the weight average molecular weight of the surface portion forming 60 to30 wt % of the particle is 1.2 to 2.2 times as large as that of thecentral portion forming 30 to 60 wt % of the particle.

4. An expandable styrene resin particle, wherein the inclination of acorrelation expression of a logarithm (R.M.S radius) and a logarithm(MW), measured by a GPC/MALLS method, of a surface portion from thesurface of the particle to a depth of ⅕ of the radius toward the centeris not larger than 0.53.

Further, according to another embodiment of the process of the presentinvention, in suspension polymerization of styrene monomers, theconcentration of oxygen in a reaction vessel is kept at 1 vol % or lowerduring from the start of polymerization to the addition of additionalstyrene monomers in the course of polymerization at a polymerizationrate of 60% or higher.

According to this method, the following expandable styrene resinparticles can be obtained.

1. An expandable styrene resin particle, wherein when a surface portionfrom the surface of the particle to a depth of ⅕ of the radius towardthe center is further divided into 6 equal portions from the surfacetoward the center of the particle, the weight average molecular weightsof parts constituting from the surface to the ⅙ to 6/6 portions do notdecrease toward the surface of the particle.

2. The particle of the paragraph 1, wherein a ratio (B)/(A)×100(%) is atleast 130

wherein (B) is the weight average molecular weight of the outermostportion out of the 6 equal portions, and (A) is the weight averagemolecular weight of the whole particle.

3. An expandable styrene resin particle, wherein a chart of gelpermeation chromatography of a surface portion from the surface of theparticle to a depth of ⅕ of the radius toward the center has two crestsor a shoulder, and

a ratio (B)/(A)×100(%) is at least 130

wherein (B) is the weight average molecular weight of an outermostportion out of 6 equal portions obtained by dividing the surface portioninto the 6 equal portions from the surface toward the center of theparticle, and (A) is the weight average molecular weight of the wholeparticle.

4. An expandable styrene resin particle obtained bysuspension-polymerizing styrene monomers and impregnating a styreneresin particle with an easily evaporating expanding agent before orafter completion of the polymerization, wherein

a ratio (B)/(A)×100(%) is larger than 130 but not larger than 200

wherein (B) is the weight average molecular weight of a resin componentforming up to 10 wt % from the surface toward the center of theparticle, and (A) is the weight average molecular weight of the wholeparticle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining a method of measuring the molecularweights of 5 equally divided portions of a particle.

FIG. 2 is a diagram for explaining a method of measuring the molecularweights of 6 equally divided portions of the outermost portion out of 5equally divided portions of a particle.

FIG. 3 is a diagram for explaining a method of measuring the molecularweight of an expandable cell in the surface of a particle.

FIG. 4 is a graph showing changes in molecular weights from the centerto the surface of particles obtained in Example 1 and ComparativeExample 1.

FIG. 5 is GPC charts of particles obtained in Examples 1 and 2 andComparative Example 1.

FIG. 6 is a graph showing changes in molecular weights from the centerto the surface of particles obtained in Examples 10, 13 and 14 andComparative Example 1.

FIG. 7 is GPC charts of particles obtained in Examples 13 and 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The expandable styrene resin particles of the present invention can beobtained by polymerizing styrene monomers. As the styrene monomers,styrene itself or mixed monomers comprising styrene as an essentialcomponent, and a styrene derivative such as a-methylstyrene,chlorstyrene or vinyltoluene, an acrylic ester such as methyl acrylate,ethyl acrylate or butyl acrylate, or a methacrylic ester such as methylmethacrylate, ethyl methacrylate or butyl methacrylate can be used.Further, a crosslinking agent such as divinylbenzene or diallylphthalate can be used.

A process for producing the expandable styrene resin particles ispreferably suspension polymerization, and a conventionally known processcan be employed. The polymerization is generally carried out bydispersing styrene monomers having a catalyst such as an organicperoxide dissolved therein into an aqueous medium containing adispersant so as to produce radicals.

As the dispersant, a hardly soluble inorganic salt and a surfactant maybe used in combination or a conventionally known dispersant such as anorganic dispersant, e.g., PVA can also be used.

As the hardly soluble inorganic salt, magnesium phosphate, tricalciumphosphate and the like can be used. As the surfactant, sodium oleate,sodium dodecylbenzenesulfonate as well as an anionic surfactant and anonionic surfactant which are generally used in suspensionpolymerization can be used. As the organic dispersant, a polyvinylalcohol, a polyvinyl pyrrolidone, methylcellulose and the like can beused.

As the organic peroxide, a conventionally known organic peroxide havinga 10-hour half decomposition temperature of 50 to 100° C. can be used.For example, lauroyl peroxide, benzoyl peroxide, t-butyl peroxybenzoate,t-butylperoxyisopropyl carbonate and the like can be used. The organicperoxide is preferably used in an amount of 0.001 to 0.5 wt % based onthe polymerizable monomer. One or two or more organic peroxides can beused.

The molecular weight of the whole resin particles can be controlled byadjustment of the concentration of the catalyst, the use of a chaintransfer agent, or both of these.

As the chain transfer agent, conventionally known chain transfer agentssuch as octyl mercaptan, dodecyl mercaptan, an α-methylstyrene dimer andthe like can be used. The chain transfer agent is preferably used in anamount of 20 to 100 ppm based on the polymerizable monomers.

In the process of producing the present invention, a reaction is allowedto proceed, keeping the concentration of oxygen in a reaction vessel lowat least in the late stage of polymerization, and the resulting styreneresin particles are impregnated with an easily evaporating expandingagent before or after completion of the polymerization reaction.

In this process, the concentration of oxygen in the reaction vessel maybe kept low from the start or in the middle of the polymerization, andthe concentration of oxygen must be low at least in the late stage ofthe polymerization.

In general, when the polymerization proceeds with oxygen present in thereaction vessel, the amount of low-molecular-weight products formed instyrene resin particles increases. Particularly, since the amount ofresidual polymerization catalyst is small and radicals are stopped inthe late polymerization stage, the low-molecular-weight products areliable to be produced on the surface of the styrene resin particles,thereby impairing the appearance of a molded article.

Meanwhile, in the production process of the present invention, since theconcentration of oxygen in the reaction vessel is kept low in the latepolymerization stage, the occurrence of such low-molecular-weightproducts can be inhibited. The concentration of oxygen is kept atpreferably 7 vol % or lower, more preferably 5 vol % or lower,particularly preferably 1 vol % or lower. The concentration of oxygencan be controlled by substitution with an inert gas such as nitrogen.

Further, the late polymerization stage is a stage with a polymerizationrate of preferably not lower than 60%, more preferably not lower than60% but lower than 97%.

According to a first production process of the present invention, insuspension polymerization, additional styrene monomers are added at apolymerization rate of not lower than 60%, preferably not lower than 60%but lower than 97%, while the concentration of oxygen in a reactionvessel is kept at 7 vol % or lower.

If the styrene monomers are added while the concentration of oxygen ishigher than 7 vol %, low-molecular-weight products may be produced inthe surface layer of styrene resin particles. The low-molecular-weightproducts produced in the surface promote thermal fusion at the time ofheat-expand-molding excessively, thereby. degrading the strength andsurface finish of a molded article.

Further, according to a second production process of the presentinvention, in suspension polymerization, the concentration of oxygen ina reaction vessel is kept at 1 vol % or lower during from the start ofpolymerization to a polymerization rate of not lower than 60%,preferably not lower than 60% but lower than 97%, and additional styrenemonomers are added at the polymerization rate of not lower than 60%,preferably not lower than 60% but lower than 97%.

In this case, before the polymerization starts, oxygen in the reactionvessel is substituted with nitrogen or the like so as to adjust theconcentration of oxygen in the reaction vessel to 1 vol % or lower inadvance. The lower the concentration of oxygen is, the more preferableit is. Thereafter, to allow the polymerization to proceed, feeding ofnitrogen or the like into the reaction vessel may be continued or thereaction vessel may be sealed after completion of substitution so as toprevent oxygen from entering the reaction vessel.

When the polymerization proceeds in a sealed reaction vessel, oxygencontained in water and styrene monomers, oxygen trapped in a liquid byagitation and oxygen produced at the time of the polymerization reactionare generated along with an increase in the temperature of thepolymerization reactants from a feed temperature to a reactiontemperature or along with a reaction of a polymerization catalyst.Hence, when the concentration of oxygen exceeds 1 volt, oxygen issubstituted with nitrogen or the like again.

The concentration of oxygen in the reaction vessel is controlled to beat 1 vol % or lower until addition of an additional styrene monomers iscompleted. To keep the concentration of oxygen at 1 vol % or lowerduring the polymerization, the connection of an oximeter is recommended.

When the concentration of oxygen is 1 vol % or lower, the molecularweight becomes further higher, whereby the strength of a molded articlecan be rendered high.

When the polymerization rate is lower than 60%, absorption of thestyrene monomers into the styrene resin particles is promoted, and themolecular weight of the central portion becomes high, whereby anexpanding force and fusion of the molded article may deteriorate.Meanwhile, when the polymerization rate is equal to or higher than 97%,absorption of the styrene monomers into the resin particles lowers, theamounts of radicals and polymerization catalyst in the resin particlesdecrease, and the molecular weight of the outermost portion of resinparticles becomes low, so that thermal fusion at the time ofexpand-molding is promoted excessively, whereby the strength and surfacefinish of the molded article may be degraded. Addition of the monomersat a polymerization rate of not lower than 85% but lower than 97% ismore preferable.

The styrene monomers are preferably added in an amount of 5 to 30 wt %,more preferably 10 to 15 wt %, based on finally obtained styrene resinparticles.

When the amount is smaller than 5 wt %, the molecular weight of theoutermost portion of the styrene resin particles may not be sufficientlyhigh. As a result, the strength may not be improved sufficiently.Meanwhile, when the amount is larger than 30 wt %, the resin particlesmay be softened, absorption of the styrene monomers may be promoted, andthe molecular weight of the central portion of particles may becomehigher, i.e., a part with the highest molecular weight may be closer tothe central portion of the particles, so that an expanding force lowersand an molded article may hardly be fused accordingly.

The suspension polymerization temperature is generally 80 to 95° C. Theadditional styrene monomers may be added at the temperatures or athigher temperatures. In consideration of the industrial productionefficiency that the amount of styrene monomers remaining in the finallyobtained expandable styrene resin particles is reduced, thepolymerization temperature is preferably not lower than 90° C., and thestyrene monomers are preferably added during temperature increasing.

According to a third production process of the present invention, insuspension polymerization, a reaction is allowed to proceed, keeping theconcentration of oxygen in a reaction vessel at 7 vol % or lower fromthe start of polymerization, and no styrene monomers are added in themiddle of the polymerization.

In the production process of the present invention, it is preferred thatthe polymerization be started when a hydrogen ion concentration in anaqueous dispersion is 8 to 10 and an additional hardly soluble inorganicsalt be added at least once at a polymerization rate of 20 to 50%. Theaqueous dispersion is preferably a continuous phase.

When the hydrogen ion concentration is out of the above range, particlesize distribution at the completion of the suspension polymerization maynot be sharp. The hydrogen ion concentration can be adjusted by use of abasic inorganic salt.

Further, due to the same reason, an additional hardly soluble inorganicsalt can be added at a polymerization rate of 20 to 50%.

The hardly soluble inorganic salt can be additionally added at leastonce, e.g., two or three times. Further, the hardly soluble inorganicsalt can be additionally added after the polymerization furtherproceeds.

An easily evaporating expanding agent can be added under pressure alongwith addition of the styrene monomers. However, in general, the easilyevaporating expanding agent is preferably added after addition of thestyrene monomers, and it is preferred that the styrene resin particlesbe impregnated with the easily evaporating expanding agent before orafter completion of a polymerization reaction.

The easily evaporating expanding agent is selected from aliphatichydrocarbons such as propane, isobutane, normal butane, isopentane,normal pentane and cyclopetane. Further, as an expanding assistant, analicyclic or aromatic hydrocarbon such as cyclohexane can be used incombination with the easily evaporating expanding agent in addition toan aliphatic hydrocarbon.

In the polymerization, additives used in production of expandablestyrene resin particles, such as a solvent, a plasticizer, an expandablecell nucleating agent, a filler, a flame retardant, a flame retardantassistant, a lubricant and a colorant may be used as required.

Further, in the production process of the present invention, a seedpolymerization process using expandable styrene resin particles orregenerated styrene resin particles as seeds can also be employed. Inthis process as well, the concentration of oxygen is controlled to below, as described above.

After impregnated with the expanding agent, discharged from thepolymerization system and dried through dehydration, the expandablestyrene resin particles may be coated with a surface coating agent asrequired. As the coating agent, a conventionally known coating agentwhich has been applied to expandable styrene resin particles can beemployed. Illustrative examples of such a coating agent include zincstearate, triglyceride stearate, monoglyceride stearate, a castorhardened oil, an amide compound, silicones, and an antistatic agent.

In general, the weight average molecular weight (molecular weight) ofthe expandable styrene resin particles produced by suspensionpolymerization is determined by the amount of polymerization catalyst,and the molecular weights of the central, middle and surface-layerportions of particles are approximately the same.

However, according to the foregoing first production process of thepresent invention, expandable styrene resin particles whose surfaceportions have a higher molecular weight than these central portions areobtained. The gradient of the molecular weight from the center to thesurface of particles is not a gradual increase at a given rate, but themolecular weight sharply increases near the surface.

The expandable styrene resin particles obtained by the productionprocess of the present invention has a much higher molecular weight nearthe surface than the rest of the particle. Therefore, it can have a highmolecular weight in the surface portion with the molecular weight of thecentral portion kept low. In general, when the central portion has a lowmolecular weight, good expandability can be exhibited, while when thesurface portion has a high molecular weight, a molded article has highstrength. Thus, the particles of the present invention can satisfy bothexpandability and the strength of a molded article. For example, amolded article having relatively high strength can be obtained with acertain degree of expandability maintained.

In particular, the expandable styrene resin particles can be obtained inwhich the weight average molecular weight of a surface portion from thesurface of the particles to a depth of ⅕ of the radius toward the centeris higher than that of a central portion from the center to a distanceof ⅕ of the radius toward the surface.

Next, the surface portion and the central portion will be described withreference to FIG. 1. As shown in FIG. 1, a resin particle 10 is dividedinto 5 equal portions from the surface toward the center. An outermostportion 1 out of the 5 portions is the surface portion. The weightaverage molecular weight of the surface portion is that of the portion1. An innermost portion 5 out of the 5 portions is the central portion.The weight average molecular weight of the central portion is that of amiddle portion out of 5 equally divided portions of the portion 5.

Further, it is preferred that a chart of gel permeation chromatographyof the surface portion have two crests or a shoulder. Having the twomountains or shoulder indicates that the molecular weight is changingabruptly. The shoulder is formed by an inflection point. In the presentinvention, the chart by gel permeation chromatography is measured by useof two GL-R400M columns of Hitachi Chemical Co., Ltd. Referring to FIGS.5 and 7, although the chart generally has inflection points at bothlower ends as well, the present invention does not count these points asshoulders.

Further, in the present invention, it is preferred that the weightaverage molecular weight of the central portion be 200,000 to 300,000,the weight average molecular weight of the surface portion be 300,000 to450,000 and the weight average molecular weight of the surface portionbe at least 1.2 times as large as that of the central portion.

When the molecular weight of the central portion is smaller than200,000, the strength of a molded article may be low. Further, to renderthe molecular weight smaller than 200,000, the amount of catalyst usedin a production process must be increased disadvantageously.

Meanwhile, when the molecular weight of the central portion is largerthan 300,000, expandability may be low.

Further, the weight average molecular weight of the central portion ispreferably 200,000 to 250,000. The molecular weights of the threeinnermost portions out of the 5 portions are preferably substantiallythe same.

When the molecular weight of the surface portion is smaller than300,000, a molded article may not be able to have sufficient strength.

When the molecular weight of the surface portion is larger than 450,000,an expanding force may lower, thermal fusion may not proceed, thesurface finish of the molded article may become poor, and fusion hardlyoccurs.

Further, the weight average molecular weight of the surface portion ispreferably 350,000 to 450,000.

The ratio of the weight average molecular weight of the surface portionto that of the central portion is more preferably at least 1.5,generally at most 2.2.

Further, according to the production process of the present invention,grafting which has been conceived not to occur in conventional radicalpolymerization of styrene occurs in the surface portion, whereby ahigh-molecular-weight branching structure can be produced.

It can be known that the surface portion has the branching structure,because the inclination of a correlation expression of a logarithm(R.M.S radius) and a logarithm (MW), measured by a GPC/MALLS method, ofthe surface portion from the surface of the particle to a depth of ⅕ ofthe radius toward the center is not larger than 0.53, preferably notlarger than 0.52, more preferably not larger than 0.50. In the abovedescription, GPC stands for gel permeation chromatography, MALLS standsfor Multi Angle Laser Light Scattering, an R.M.S radius refers to a RootMean Square radius, and MW refers to an absolute molecular weight.

The above inclination is 0.55 to 0.60 for a linear polystyrene obtainedby general radical polymerization (suspension-based).

Further, due to the same reason as above, the weight average molecularweight of the surface portion is preferably 300,000 to 450,000.

Further, according to the foregoing second production process of thepresent invention, a decrease in molecular weight at the end of apolymerization reaction, that is, in a portion near the surface, can beprevented by keeping the concentration of oxygen at 1 vol % or lowerfrom the start of the polymerization to the late stage of thepolymerization.

In particular, in the expandable styrene resin particles of the presentinvention, when a surface portion from the surface of a particle to adepth of ⅕ of the radius toward the center is further divided into 6equal portions from the surface toward the center of the particle, theweight average molecular weights of parts constituting from the surfaceto the ⅙ to 6/6 portions preferably do not decrease toward the surfaceof the particle, more preferably increase toward the surface thereof.

Next, the “parts constituting from the surface to the ⅙ to 6/6 portions”will be described with reference to FIG. 2. As shown in FIG. 2( a),firstly, a resin particle 10 is halved, and then a half thereof isdivided into 5 equal portions from the surface toward the center. Anoutermost portion A out of the 5 equal portions is further divided into6 equal portions as shown in FIG. 2( b). The above “parts constitutingfrom the surface to the ⅙ to 6/6 portions” are the parts from thesurface to these 6 portions respectively.

In the present invention, it is preferred that the weight averagemolecular weight (B) of the outermost portion out of the 6 equalportions be larger than that (A) of the whole resin particle. Inparticular, a ratio (B)/(A)×100(%) is more preferably at least 130.Further, the value is generally 200 or less.

In the present invention, the outermost portion out of the 6 equalportions generally corresponds to a resin component forming up to 10 wt% from the surface toward the center of the particle.

By making the molecular weight of the outermost portion relativelyhigher, the strength of a molded article can be further increased.

In the resin particles produced by the first and second productionprocess, as described above, the occurrence of low-molecular-weightproducts in the surface portion is inhibited, and the surface portionhas a high-molecular-weight branching structure. Hence, as compared witha foamed article produced by ordinary polymerization which does notcontrol the concentration of oxygen, the heat resistance of the surfaceportion is improved, and a foamed article having a good appearance orgood mechanical strength can be obtained.

Further, according to the third production process, expandable styreneresin particles in which the molecular weight of outermost portion doesnot decrease, even in terms of sizes of expandable cells in expandablestyrene beads can be obtained. As a result, a foamed article with goodappearance can be obtained.

The average particle diameter of the expandable styrene resin particlesof the present invention is generally 0.05 to 2.0 mm.

Expandable beads of the present invention are produced by expanding theexpandable styrene resin particles. Further, a foamed article of thepresent invention is produced by molding the above expandable beads.

In general, the expandable styrene resin particles are heated by steamor the like to be pre-foamed to a given bulk density and then subjectedto an aging step to produce the expandable beads. Thereafter, theexpandable beads are filled in a mold and foamed under heating again,thereby producing the expand-mold product.

In the present invention, the expandability of the expandable styreneresin particles and the strength of a molded article obtained therefromare well-balanced. The molded article of the present invention can besuitably used in food containers, packing materials, cushioningmaterials, and the like.

According to the present invention, expandable styrene resin particleswhich can produce a molded article having high strength and hasexcellent expandability, expandable beads and a foamed article can beprovided.

EXAMPLES

Methods of evaluating properties in examples and comparative examplesare as follows.

(1) Weight Average Molecular Weight (Molecular Weight)

The molecular weight of expandable styrene resin particles was measuredafter expanding the particles. The expandable styrene resin particleswere expanded in saturated steam to a bulk density of 80 ml/g.

<Method of Measuring Molecular Weights of 5 Equally Divided Portions ofParticle>

Two or three expanded particles were picked. A half of a particle 1 wasdivided into 5 equal portions by use of a razor as shown in FIG. 1 so asto form portions 1, 2, 3, 4 and 5 from the surface. The molecular weightof the outermost portion 1 (surface portion) was measured as it was. Asfor the innermost portion 5 (central portion), the portion was dividedinto 5 equal portions, a middle portion was hollowed out by use of aninjection needle, and the molecular weight of the hollowed portion wasmeasured. As for the portion 3 (corresponding to ⅗ portion from thecenter), a middle portion was hollowed out by use of an injection needleas in the case of the portion 5, and the molecular weight of thehollowed portion was measured.

The molecular weights were measured in accordance with a gel permeationchromatography (GPC) method by use of the following devices andconditions. Further, a chart (GPC chart) of the surface portion wasobtained.

-   Measuring Device: manufactured by HITACHI CO., LTD.-   Eluant: THF, Flow Rate: 2 ml/min-   Detector: UV 220 nm-   Column: Two GL-R400M columns of HITACHI CHEMICAL CO., LTD.    <Method of Measuring Molecular Weights of 6 Equally Divided Portions    of Surface Portion Out of 5 Equally Divided Portions of Particle    (Examples 10 to 16 and Comparative Examples 1 and 4)>

The molecular weights of “parts constituting ⅙ to 6/6 from the surface”were measured in the following manner. As shown in FIG. 2( a), firstly,an expanded particle 10 was halved, and a half thereof was furtherdivided into 5 equal portions from the surface toward the center. Anoutermost portion A from the surface to a depth of ⅕ was further cutinto 6 equal portions under a microscope as shown in FIG. 2( b) so as toobtain portions a, b, c, d, e and f. The molecular weights of theportions a, b, c, d, e and f were measured. The molecular weight of theportion a was the molecular weight of a portion from the surface to adepth of ⅙. An average of the molecular weights of the portions a and bwas the molecular weight of the portion from the surface to a depth of2/6. An average of the molecular weights of the portions a, b and c isthe molecular weight of the portion from the surface to a depth of 3/6.An average of the molecular weights of the portions a, b, c and d is themolecular weight of the portion from the surface to a depth of 4/6. Anaverage of the molecular weights of the portions a, b, c, d and e is themolecular weight of the portion from the surface to a depth of ⅚. Anaverage of the molecular weights of the portions a, b, c, d, e and f isthe molecular weight of the portion from the surface to a depth of 6/6.

The molecular weights were measured by a GPC method as in the abovecase.

Method of Measuring Molecular Weight of Expandable Cell in Surface ofParticle (Examples 17 and 18 and Comparative Example 5)>

5 or 6 expandable beads (average particle diameter: 3.0 mm) wereprepared and halved. Then, as shown in FIG. 3, a layer portion X and alayer portion Y which had the same thickness corresponding to the sizeof one expandable cell were cut from the surface of a halved expandablebead toward its center under a microscope. The molecular weights of theoutermost layer portion (layer portion X) and a surface layer portion(layer portion X+layer portion Y) were measured. The outermost layerportion is a portion having a thickness corresponding to the size of oneexpandable cell, and the surface layer portion is a portion having athickness corresponding to the size of two expandable cells. Themolecular weight of the surface layer portion was measured as an averageof the molecular weights of the layer portions X and Y.

In the present examples, the size of one expandable cell of theoutermost layer portion is about 50 to 100 μm, and the size of twoexpandable cells of the surface layer portion is about 100 to 200 μm.

The molecular weights were measured by a GPC method as in the abovecase.

(2) Expandability

To determine expandability, there was measured a bulk density (degree ofexpansion) when expandable styrene resin particles containing 7.0 wt %of an evaporating component were expanded in boiling water at 100° C.for 3 minutes.

(3) Flexural Strength

Expandable styrene resin particles were expanded by use of an HBP-700expanding machine of HITACHI TECHNOPLANT CO., LTD. to obtain expandablebeads. Then, the expandable beads were molded by use of a VS-500 moldingmachine of DAISEN KOUGYO CO., LTD. at a steam pressure of 0.08 MPa toobtain a molded article having a size of 550 mm×335 mm×150 mm.

The flexural strength of the foamed article having a density of 60 ml/gwas measured in accordance with JIS-A-9511.

(4) Analysis of polymer structure of surface portion by GPC/MALLS method

A surface portion 1 shown in FIG. 1 was used as a sample to be measured.The GPC/MALLS method was carried out by use of the following devices andconditions. In consequence, the inclination of a correlation expressionof a logarithm (R.M.S radius) and a logarithm (MW) was determined.

-   Column: Shodex, KF-807L (×2)-   Column Temperature: 40° C.-   Eluant: THF-   Flow Rate: 1.00 ml/min-   Amount of Injection: 100 μL-   Detector: RI and Wyatt Technology, DAWN DSP-F

(Laser Wavelength: 632.8 nm)

-   Multi Angle Fitting Method: Berry Method

(5) Appearance (Smoothness of Surface)

Black printing ink was applied thinly on the surface of a molded articleproduced in the same manner as in (3) by use of a roller, and theapplied surface portion was subjected to an image processor. Since voidson the surface portion were not coated with the ink, the area of theblack portion with respect to the whole applied area was determined as adegree of smoothness of the surface and a value for evaluating theappearance of the molded article.

(6) Rate of Polymerization

A rate of polymerization was measured by sampling resin particles duringsynthesis by use of the following devices and conditions.

-   Measuring Device: manufactured by HITACHI CO., LTD.-   Eluant: acetonitrile/distilled water=70/30-   Flow Rate: 1 ml/min-   Detector: UV 230 nm-   Column: Inertsil ODS-2    <Examples of keeping concentration of oxygen at 7 vol % or lower    only in late stage of polymerization and adding additional monomers    in late stage of polymerization>

Example 1

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 5,400 g of styrene, 22.0 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed. Thevessel was then heated to 90° C., and 3 g of tricalcium phosphate wasadded two hours and three hours after completion of temperature rising,respectively. At these times, rates of polymerization were 40% and 46%,respectively.

Then, upon keeping the content of the vessel at 90° C. for 2 hours, 6 gof tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonate wereadded again. A rate of polymerization at this time was 95%. The insideof the polymerization vessel was substituted with nitrogen at a rate of200 to 300 ml/min for 10 minutes. The concentration of oxygen in thepolymerization vessel at this time was 3.1 vol %. Thereafter, 600 g ofstyrene was added dropwise continuously over 3 hours while thetemperature was raised to 100° C.

Then, 90 g of cyclohexane was added under pressure, and after 1 hour,420 g of butane (isobutane/normal butane ratio=4/6) was added underpressure in 1 hour, and then the content of the vessel was kept foranother 4 hours. Thereafter, the content was cooled to room temperatureand then taken out of the autoclave.

After the taken out slurry was washed, dehydrated and dried, theresulting product was classified by passing through a 14 mesh and beingcaught in a 26 mesh and then coated with 0.08% of zinc stearate, 0.05%of castor hardened oil and 0.02% of dimethyl silicone to obtainexpandable styrene resin particles.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 1.Further, a change in molecular weight from the center toward the surfaceis shown in FIG. 4.

In addition, a chart (GPC chart) of a surface portion was measured by aGPC method. The chart is shown in FIG. 5( a).

Example 2

After the inside of a 14-liter autoclave equipped with an agitator wassubstituted with nitrogen at a rate of 500 to 600 ml/min for 30 minutes,6,000 g of pure water, 9 g of tricalcium phosphate and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 5,400 g of styrene, 23.6 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed, apipe for blowing was opened, and then nitrogen was flown at a rate of200 to 300 ml/min. The vessel was then heated to 90° C., and 3 g oftricalcium phosphate was added 1.5 hours and 2.5 hours after completionof temperature rising, respectively. At these times, rates ofpolymerization were 39% and 46%, respectively.

Then, upon keeping the content of the vessel at 90° C. for 2 hours, 6 gof tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonate wereadded again. Up to this point, nitrogen was flown continuously. At thistime, a rate of polymerization was 96% and the concentration measuredwas 0.1 vol %. After the concentration of oxygen was measured, the flowof nitrogen was stopped, the pipe for blowing was closed, and then 600 gof styrene was added dropwise continuously over 3 hours while thetemperature was raised to 100° C.

The procedures thereafter including impregnation with an expanding agentwere repeated in the same way of Example 1.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 1.

In addition, a chart (GPC chart) of a surface portion was measured by aGPC method. The chart is shown in FIG. 5( b).

Comparative Example 1

The molecular weight and properties of expandable styrene resinparticles (trade name: S-HCM-K, product of Shonan Sekisui Kougyo Co.,Ltd.) were measured, and the results are shown in Tables 1 and 3.Further, a change in molecular weight from the surface toward the centeris shown in FIG. 4.

In addition, a chart (GPC chart) of a surface portion was measured by aGPC method. The chart is shown in FIG. 5( c).

As shown in FIG. 4, the molecular weights of both the particles ofExample 1 and Comparative Example 1 were increased from the centertoward the surface. However, while the molecular weight of the particlesof Comparative Example 1 was increased gradually, the molecular weightof the particles of Example 1 remained almost uniform from the center to⅗ from the center and became abruptly high near the surface. Thus, it isunderstood that the molecular weight near the surface of the particlesof Example 1 is high while its molecular weight near the center is keptlow.

Further, as shown in FIGS. 5( a), 5(b) and 5(c), the GPC charts of theparticles of Examples 1 and 2 with an abrupt change in molecular weighthad a shoulder. Inflection points are present at these shoulders. Theshoulders are formed because the proportion of high molecular weightpolymers is high. Meanwhile, in the GPC chart of the particles ofComparative Example 1 showing a gradual change in molecular weight, asmall bulge is seen. However, the chart with no inflection points,indicating that no shoulders were formed.

Example 3

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 5,400 g of styrene, 20.4 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed. Thevessel was then heated to 90° C., and 3 g of tricalcium phosphate wasadded two hours and three hours after completion of temperature rising,respectively. At these times, rates of polymerization were 35% and 44%,respectively.

Then, upon keeping the content of the vessel at 90° C. for 2 hours, 6 gof tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonate wereadded again. A rate of polymerization at this time was 91%. After theinside of the polymerization vessel was substituted with nitrogen at arate of 200 to 300 ml/min for 10 minutes, the concentration of oxygenwas measured. It was 4.8 vol %. Thereafter, 600 g of styrene was addeddropwise continuously over 3 hours while the temperature was raised to100° C.

The remaining procedure of Example 1 including impregnation with anexpanding agent was repeated.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 1.

Example 4

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 5,700 g of styrene, 20.4 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed. Thevessel was then heated to 90° C., and 3 g of tricalcium phosphate wasadded two hours and three hours after completion of temperature rising.At these times, rates of polymerization were 35% and 44%, respectively.

Then, upon keeping the content of the vessel at 90° C. for 2 hours, 6 gof tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonate wereadded again. A rate of polymerization at this time was 90%. After theinside of the polymerization vessel was substituted with nitrogen at arate of 200 to 300 ml/min for 10 minutes, the concentration of oxygenmeasured was 4.5 vol %. Thereafter, 300 g of styrene was added dropwisecontinuously over 3 hours while the temperature was raised to 100° C.

The procedures thereafter including impregnation with an expanding agentwere repeated in the same way of Example 1

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 1.

Example 5

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 5,400 g of styrene, 20.0 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation. After completion of charging, thepolymerization vessel was sealed. The vessel was then heated to 90° C.,and 3 g of tricalcium phosphate was added two hours and three hoursafter completion of temperature rising, respectively. At these times,rates of polymerization were 34% and 43%, respectively.

Then, upon keeping the content of the vessel at 90° C. for 2 hours, 6 gof tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonate wereadded again. A rate of polymerization at this time was 90%. After theinside of the polymerization vessel was substituted with nitrogen at arate of 200 to 300 ml/min for 10 minutes, the concentration of oxygenwas measured. It was 4.0 vol %. Thereafter, 600 g of styrene was addeddropwise continuously over 1.5 hours while the temperature was raised to100° C.

The procedures thereafter including impregnation with an expanding agentwere repeated in the same way of Example 1.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 1.

Example 6

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 5,400 g of styrene, 22.4 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation. After completion of charging, thepolymerization vessel was sealed. The vessel was then heated to 90° C.,and 3 g of tricalcium phosphate was added 2 hours and 2.5 hours aftercompletion of temperature rising, respectively. At these times, rates ofpolymerization were 38% and 43%, respectively.

Then, upon keeping the content of the vessel at 90° C. for 0.5 hours, 6g of tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonatewere added again. A rate of polymerization at this time was 61%. Afterthe inside of the polymerization vessel was substituted with nitrogen ata rate of 200 to 300 ml/min for 10 minutes, the concentration of oxygenwas measured. It was 4.1 vol %. Thereafter, 600 g of styrene was addeddropwise continuously over 5 hours while the temperature was raised to100° C.

The procedures thereafter including impregnation with an expanding agentwere repeated in the same way of Example 1.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 1.

Example 7

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 4,200 g of styrene, 21.7 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation. After completion of charging, thepolymerization vessel was sealed. The vessel was then heated to 90° C.,and 3 g of tricalcium phosphate was added 1.5 hours and 2 hours aftercompletion of temperature rising, respectively. At these times, rates ofpolymerization were 35% and 40%, respectively.

Then, upon keeping the content of the vessel at 90° C. for 1.5 hours, 6g of tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonatewere added again. A rate of polymerization at this time was 96%. Afterthe inside of the polymerization vessel was substituted with nitrogen ata rate of 200 to 300 ml/min for 10 minutes, the concentration of oxygenwas measured. It was 3.8 vol %. Thereafter, 1,800 g of styrene was addeddropwise continuously over 6 hours while the temperature was raised to100° C.

The procedures thereafter including impregnation with an expanding agentwere repeated in the same way of Example 1.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 1.

Example 8

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 5,400 g of styrene, 22.4 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation. After completion of charging, thepolymerization vessel was sealed. The vessel was then heated to 90° C.,and 3 g of tricalcium phosphate was added 2 hours and 2.5 hours aftercompletion of temperature rising, respectively. At these times, rates ofpolymerization were 38% and 43%, respectively.

Then, upon keeping the content of the vessel at 90° C. for 0.5 hours, 6g of tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonatewere added again. A rate of polymerization at this time was 61%. Afterthe inside of the polymerization vessel was substituted with nitrogen ata rate of 200 to 300 ml/min for 10 minutes, the concentration of oxygenwas measured. It was 6.5 vol %. Thereafter, 600 g of styrene was addeddropwise continuously over 5 hours while the temperature was raised to100° C.

The procedures thereafter including impregnation with an expanding agentwere repeated in the same way of Example 1.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 1.

Example 9

(Production of Seeds Comprising Regenerated Styrene Resin Particles)

Foamed styrene articles (foamed article obtained from HIGHBEAD SSB-HX ofHITACHI CHEMICAL CO., LTD.) were shrunk by hot air at 220° C. to obtaina shrunk article having an apparent specific gravity of 0.8, a size of500 mm×400 mm×100 mm and a weight of about 16 kg. The shrunk article wascoarse-grained by use of a grinder (trade name: ZA-560 type grinder,product of HORAI CO., LTD.) equipped with a 10-mm screen. The thusobtained coarse grains had a maximum length of about 10 mm and a bulkspecific density of 0.65. Then, in a Henschel mixer (product of MITSUIMIIKE KAKOU CO., LTD., FM10B), 2,000 g of the coarse grains, 20 g oftalc (product of HAYASHI KASEI CO., LTD., MICROWHITE #5,000) having anaverage particle diameter of 10 μm, and 0.6 g of ethylenebisstearylamidewere charged, and they were mixed at 2,000 rpm for 2 minutes. The coarsegrains coated with talc and ethylenebisstearylamide were melt-extrudedby use of a 30-mm vented extruder (T-shaped die, width of sheet: 300 mm,thickness of sheet: 1 mm), the sheet being pulled at the substantiallysame rate as the extrusion rate. Further, before the sheet was cooledand solidified, slits each having a depth of 0.5 mm were made in thesheet at intervals of 1 mm horizontally to the direction of extrusion byuse of a roller. After the sheet was cooled and solidified, it was cutinto about 10 to 15 cm by use of a cutter. Subsequently, the sheet-likecut pieces of styrene resin obtained were fine-grained by means of agrinder (trade name: VM-16 type grinder, product of ORIENT CO., LTD.)equipped with a 2 mm screen. The fine grains were classified into arange of 0.6 to 1.2 mm by use of a sieve to obtain regenerated styreneresin particles.

The regenerated styrene resin particles had a weight average molecularweight of 169,000 and a specific gravity of 0.91.

(Production of Regenerated Expandable Styrene Resin Particles)

In a 5-liter pressure agitation vessel, 1,900 g of deionized water,1,100 g of the regenerated styrene resin particles (seeds) obtained,12.0 g of tricalcium phosphate, and 0.09 g of sodiumdodecylbenzenesulfonate were charged, and the mixture was heated to 75°C. under agitation.

Then, 400 g of deionized water and 1.3 g of polyvinyl alcohol werecharged into a monomer dispersion vessel and mixed together. To themixture, 200 g of styrene monomer having 0.2 g of t-butyl peroxide and3.9 g (Wet %) of benzoyl peroxide dissolved therein was added. Theresulting mixture was agitated by use of a homomixer (product of TOKUSHUKIKA KOUGYO CO., LTD.) at 5,800 rpm for 120 seconds to finely dispersethe styrene monomers (average diameter of monomer oil drop: 10 to 100μm). The styrene monomer dispersion was charged into the vessel over 30minutes, then kept as it was for 60 minutes, and then heated to 90° C.

Thereafter, 900 g of styrene monomers was added continuously at aconstant rate (3.0 g/min) over 5 hours. In this case, the pressureagitation vessel was purged with nitrogen to keep the concentration ofoxygen at 0.5 to 1 vol %. The content of the styrene monomers at thispoint was 10% (polymerization rate: 90%).

Then, after 2.2 g of tricalcium phosphate and 0.05 g of sodiumdodecylbenzenesulfonate were added, the resulting mixture was heated to115° C. and kept at the temperature for 2 hours. Then, after the mixturewas cooled to 100° C., 90 g of butane (i/n ratio=4/6, which will remainthe same in the following description) as an expanding agent was addedunder pressure twice, and the resulting mixture was kept for 10 hours toimpregnate it with the expanding agent.

After cooled to room temperature, regenerated expandable styrene resinparticles impregnated were taken out of the vessel and dehydrated to bedried.

Then, the resin particles were classified with a sieve with openings of0.6 to 1.7 mm, and 0.1 wt % of zinc stearate and 0.1 wt % of hardenedcastor oil were added to the resulting particles for coating to obtaincoated regenerated expandable styrene resin particles.

The molecular weight and properties of the particles were measured, andthe results are shown in Table 1.

In the present example, the obtained regenerated expandable styreneresin particles were pre-foamed to 50 ml/g, aged for about 18 hours, andthen molded at a molding pressure of 0.08 MPa by use of an expandablestyrene resin molding machine VS-300 manufactured by DAISEN KOUGYO CO.,LTD. to obtain a molded article having a size of 550 mm×335 mm×150 mm.

Comparative Example 2

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 6,000 g of styrene, 20.8 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed. Thevessel was then heated to 90° C., and 3 g of tricalcium phosphate wasadded two hours and three hours after completion of temperature rising,respectively. At these times, rates of polymerization were 38% and 44%,respectively.

Then, upon keeping the content of the vessel at 90° C. for 2.5 hours, 6g of tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonatewere added again. A rate of polymerization at this time was 95%, and theconcentration of oxygen was 18.7 vol %. Thereafter, the mixture washeated to 100° C. over 1 hour.

The procedures thereafter including impregnation with an expanding agentwere repeated in the same way of Example 1.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 1.

Comparative Example 3

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 5,700 g of styrene, 24.8 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation. After completion of charging, thepolymerization vessel was sealed. The vessel was then heated to 90° C.,and 3 g of tricalcium phosphate was added one hour and two hours aftercompletion of temperature rising, respectively. At these times, rates ofpolymerization were 39% and 48%, respectively.

Then, upon keeping the content of the vessel at 90° C. for 2 hours, 6 gof tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonate wereadded again. A rate of polymerization at this time was 98%, and theconcentration of oxygen measured was 19.0 vol %. Thereafter, 300 g ofstyrene was added dropwise continuously over 1.5 hours while thetemperature was raised to 100° C.

The procedures thereafter including impregnation with an expanding agentwere repeated in the same way of Example 1.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 1.

TABLE 1 Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex.8 Ex. 9 Ex. 1 Ex. 2 Ex. 3 Conditions Rate of Polymerization 95 96 91 9090 61 96 61 90 — — 98 for (wt %) Polymeri- Concentration of 3.1 0.1 4.84.5 4.0 4.1 3.8 6.5 0.5 to 1 — 18.7 19.0 zation Oxygen in ReactionVessel (vol %) Amount of Additional 10 10 10 5 10 10 30 10 50 — 0 5Styrene Monomers (wt %) Results of Molecular Weight of 22.5 20.2 29.525.0 31.0 27.2 24.5 25.0 26.1 23.2 29.5 18.7 Evaluation Central Portion(×10⁴) Molecular Weight of 22.8 20.5 29.6 25.1 31.3 27.3 24.5 25.1 28.326.5 29.3 18.9 3/5 Portion from Center (×10⁴) Molecular Weight of 34.643.8 35.7 30.5 37.8 35.6 37.1 29.8 47.1 33.6 29.7 22.8 Surface Portion(×10⁴) Molecular Weight of 1.54 2.17 1.21 1.22 1.22 1.31 1.51 1.19 1.801.45 1.01 1.22 Surface Portion/ Molecular Weight of Central PortionExpandability (ml/g) 70 72 67 70 64 68 69 70 60 70 67 78 FlexuralStrength of 0.300 0.295 0.305 0.295 0.310 0.305 0.305 0.290 0.32 0.2950.285 0.255 Molded Article (MPa) Inclination of — 0.47 0.49 0.52 — —0.49 — 0.49 — 0.56 0.58 Correlation Expression of Logarithm (R.M.SRadius) and Logarithm (MW) Appearance (Degree — 93 90 88 — — 89 — 95 —85 82 of Smoothness of Surface) (%) Molecular Weight: Weight AverageMolecular Weight<Examples of Keeping Concentration of Oxygen Low From Start to LateStage of Polymerization and Adding Additional Monomers While KeepingConcentration of Oxygen at 7 vol % or Lower in Late PolymerizationStage>

Example 10

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 5,400 g of styrene, 22.4 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed, anda pipe for blowing was opened to perform substitution with nitrogen. Theconcentration of oxygen at this time was 12 vol %. After completion ofthe substitution with nitrogen, the pipe for blowing was closed to sealthe vessel again. The vessel was then heated to 90° C., and 3 g oftricalcium phosphate was added two hours and three hours aftercompletion of temperature rising, respectively. At these times, rates ofpolymerization were 40% and 49%, respectively.

Then, when the content of the vessel was kept at 90° C. for 2.5 hoursand a polymerization rate of 95% was achieved, 6 g of tricalciumphosphate and 0.3 g of sodium dodecylbenzenesulfonate were added again.The concentration of oxygen at this time was 3.1 vol %. Thereafter, 600g of styrene was added dropwise continuously over 3 hours while thetemperature was raised to 100° C.

Then, 90 g of cyclohexane was added under pressure, and after 1 hour,420 g of butane (isobutane/normal butane ratio=4/6) was added underpressure in 1 hour, and then the content of the vessel was kept warm foranother 4 hours. Thereafter, the content was cooled to room temperatureand then taken out of the autoclave.

After the taken out slurry was washed, dehydrated and dried, theresulting product was classified by passing through a 14 mesh and beingcaught in a 26 mesh and then coated with 0.08% of zinc stearate, 0.05%of castor hardened oil and 0.02% of dimethyl silicone to obtainexpandable styrene resin particles.

The molecular weight and properties of the obtained expandable styreneresin particle were measured, and the results are shown in Table 2.Further, a change in molecular weight from the center toward the surfaceis shown in FIG. 6.

Example 11

After the inside of a 14-liter autoclave equipped with an agitator wassubstituted with nitrogen, 6,000 g of pure water, 9 g of tricalciumphosphate and 0.3 g of sodium dodecylbenzenesulfonate were charged underan agitation of 230 rpm. A hydrogen ion concentration at this time was8.0.

Subsequently, 5,400 g of styrene, 22.4 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed, anda pipe for blowing was opened to perform substitution with nitrogenagain. When the concentration of oxygen at this time was measured by useof an oximeter, it was 5.4 vol %. The vessel was then heated to 90° C.,and 3 g of tricalcium phosphate was added two hours and three hoursafter completion of temperature rising, respectively. At these times,rates of polymerization were 40% and 49%, respectively.

Then, when the content of the vessel was kept at 90° C. for 2.5 hoursand a polymerization rate of 95% was achieved, 6 g of tricalciumphosphate and 0.3 g of sodium dodecylbenzenesulfonate were added again.The concentration of oxygen at this time was 4.8 vol %. Thereafter, 600g of styrene was added dropwise continuously over 3 hours while thetemperature was raised to 100° C.

The procedures thereafter including impregnation with an expanding agentand surface-processing were repeated. in the same way of Example 10.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 2.

Example 12

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 5,400 g of styrene, 22.4 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed, anda pipe for blowing was opened to perform substitution with nitrogen. Theconcentration of oxygen at this time was 11 vol %. After completion ofthe substitution with nitrogen, the pipe for blowing was closed to sealthe vessel again. The vessel was then heated to 90° C., and 3 g oftricalcium phosphate was added two hours and three hours aftercompletion of temperature rising, respectively. At these times, rates ofpolymerization were 40% and 49%, respectively.

Then, when the content of the vessel was kept at 90° C. for 2.5 hoursand a polymerization rate of 95% was achieved, 6 g of tricalciumphosphate and 0.3 g of sodium dodecylbenzenesulfonate were added. Theconcentration of oxygen at this time was 5 vol %. Then, after the insideof the vessel was substituted with nitrogen to reduce the concentrationof oxygen to 0.5 vol %, 600 g of styrene was added dropwise continuouslyover 3 hours while the temperature was raised to 100° C.

The procedures thereafter including impregnation with an expanding agentand surface-processing were repeated in the same way of Example 10.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 2.

TABLE 2 Ex. 10 Ex. 11 Ex. 12 Timing of Substitution of Inside ofReaction Vessel with Before Start Before Start Before Addition Nitrogenof of of Additional Polymerization Polymerization Monomers Concentrationof Initial Stage of Polymerization (vol %) 12.0 5.4 11.0 Oxygen inReaction Late Stage of Polymerization (vol %) 3.1 4.8 0.5 Vessel WeightAverage Whole Particle (A) 293,000 295,000 300,000 Molecular Weight 1/5Portion from Center 225,000 230,000 224,000 3/5 Portion from Center228,000 232,000 225,000 6/6 of 1/5 Portion from Surface 339,000 346,000351,000 5/6 of 1/5 Portion from Surface 340,000 4/6 of 1/5 Portion fromSurface 347,000 3/6 of 1/5 Portion from Surface 358,000 2/6 of 1/5Portion from Surface 373,000 352,000 372,000 1/6 of 1/5 Portion fromSurface (B) 357,000 340,000 366,000 (B)/(A) × 100% 122 115 122Expandability (ml/g) 70 70 70 Flexural Strength of Molded Article (MPa)0.295 0.295 0.300<Examples of Keeping Concentration of Oxygen at 1 vol % or Lower FromStart to Late Stage of Polymerization and Adding Additional Monomers inLate Stage of Polymerization>

Example 13

After the inside of a 14-liter autoclave equipped with an agitator wassubstituted with nitrogen, 6,000 g of pure water, 9 g of tricalciumphosphate, and 0.3 g of sodium dodecylbenzenesulfonate were chargedunder an agitation of 230 rpm. A hydrogen ion concentration at this timewas 8.0.

Subsequently, 5,400 g of styrene, 22.4 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed, anda pipe for blowing was opened to perform substitution with nitrogenagain. When the concentration of oxygen at this time was measured by useof an oximeter, it was 0.6 vol %. After the vessel was heated to 90° C.,substitution with nitrogen was performed again to adjust theconcentration of oxygen to 0.6 vol %, and 3 g of tricalcium phosphatewas added two hours and three hours after completion of temperaturerising. At these times respectively, rates of polymerization were 40%and 49%, respectively.

Then, when the content of the vessel was kept at 90° C. for 2.5 hoursand a polymerization rate of 95% was achieved, 6 g of tricalciumphosphate and 0.3 g of sodium dodecylbenzenesulfonate were added. Theconcentration of oxygen at this time was 0.6 vol %. Thereafter, 600 g ofstyrene was added dropwise continuously over 3 hours while thetemperature was raised to 100° C.

The procedures thereafter including impregnation with an expanding agentand surface-processing were repeated in the same way of Example 10.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 3.Further, a change in molecular weight from the center toward the surfaceis shown in FIG. 6.

In addition, a chart (GPC chart) of a surface portion was measured by aGPC method. The chart is shown in FIG. 7( a).

Example 14

After the inside of a 14-liter autoclave equipped with an agitator wassubstituted with nitrogen, 6,000 g of pure water, 9 g of tricalciumphosphate, and 0.3 g of sodium dodecylbenzenesulfonate were chargedunder an agitation of 230 rpm. A hydrogen ion concentration at this timewas 8.0.

Subsequently, 5,400 g of styrene, 22.4 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed, anda pipe for blowing was opened to perform substitution with nitrogenagain. When the concentration of oxygen at this time was measured by useof an oximeter, it was 0.9 volt. After the vessel was heated to 90° C.,substitution with nitrogen was performed again to adjust theconcentration of oxygen to 0.9 volt, and 3 g of tricalcium phosphate wasadded two hours and three hours after completion of temperature rising,respectively. At these times, rates of polymerization were 40% and 49%,respectively.

Then, when the content of the vessel was kept at 90° C. for 2.5 hoursand a polymerization rate of 95% was achieved, 6 g of tricalciumphosphate and 0.3 g of sodium dodecylbenzenesulfonate were added. Theconcentration of oxygen at this time was 1.0 vol %. Thereafter, 600 g ofstyrene was added dropwise continuously over 3 hours while thetemperature was raised to 100° C.

Then, 90 g of cyclohexane was added under pressure, and after 1 hour,420 g of butane (isobutane/normal butane ratio=4/6) was added underpressure in 1 hour, and then the content of the vessel was kept foranother 4 hours. Thereafter, the content was cooled to room temperatureand then taken out of the autoclave.

After the obtained slurry was washed, dehydrated and dried, theresulting product was classified by passing through a 14 mesh and beingcaught in a 26 mesh and then coated with 0.08% of zinc stearate, 0.05%of castor hardened oil and 0.02% of dimethyl silicone to obtainexpandable styrene resin particles.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 3.Further, a change in molecular weight from the center toward the surfaceis shown in FIG. 6.

As shown in FIG. 6, the molecular weights of portions near the surfacesof the particles of Examples 13 and 14 did not decrease, while themolecular weights of portions near the surfaces of the particles ofComparative Examples 1 and Example 10 decreased.

Example 15

After the inside of a 14-liter autoclave equipped with an agitator wassubstituted with nitrogen, 6,000 g of pure water, 9 g of tricalciumphosphate, and 0.3 g of sodium dodecylbenzenesulfonate were chargedunder an agitation of 230 rpm while feeding nitrogen into the vessel at300 ml/min. A hydrogen ion concentration at this time was 8.0.

Subsequently, 5,100 g of styrene, 25.6 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed, anda pipe for blowing was opened to feed nitrogen into the vessel at 300ml/min continuously. The concentration of oxygen at this time was 0.1vol % or lower. When the concentration of oxygen was measured againafter the vessel was heated to 90° C., it was 0.1 vol % or lower. Afterelapses of 1.5 hours and 2.0 hours after completion of temperaturerising, 3 g of tricalcium phosphate was added, respectively. At thesetimes, rates of polymerization were 40% and 47%, respectively.

Then, when the content of the vessel was kept at 90° C. for 2.0 hoursand a polymerization rate of 95% was achieved, 6 g of tricalciumphosphate and 0.3 g of sodium dodecylbenzenesulfonate were added. Theconcentration of oxygen at this time was 0.1 vol % or lower. Thereafter,substitution with nitrogen was stopped, the vessel was completelysealed, and 900 g of styrene was added dropwise continuously over 5hours while the temperature was raised to 100° C.

The procedures thereafter including impregnation with an expanding agentand surface-processing were repeated in the same way of Example 10.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 3.

In addition, a chart (GPC chart) of a surface portion was measured by aGPC method. The chart is shown in FIG. 7( b).

Further, as shown in FIGS. 7( a), 7(b) and 5(c), the GPC charts of theparticles of Examples 13 and 14 with an abrupt change in molecularweight had two crests or shoulders. An inflection point is present atthe shoulder. The shoulder is formed since the proportion of highmolecular weight polymers is high. Meanwhile, in the GPC chart of theparticles of Comparative Example 1 with a gradual change in molecularweight, a small bulge is seen. However, the chart shows no inflectionpoint, indicating that no shoulder was formed.

Example 16

After the inside of a 14-liter autoclave equipped with an agitator wassubstituted with nitrogen, 6,000 g of pure water, 9 g of tricalciumphosphate and 0.3 g of sodium dodecylbenzenesulfonate were charged underan agitation of 230 rpm with nitrogen fed into the vessel at 300 ml/min.A hydrogen ion concentration at this time was 8.0.

Subsequently, 5,400 g of styrene, 22.4 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed, anda pipe for blowing was opened to feed nitrogen into the vessel at 300ml/min continuously. The concentration of oxygen at this time was 0.7vol %. After the vessel was heated to 90° C., substitution with nitrogenwas performed again to adjust the concentration of oxygen to 0.7 vol %,and 3 g of tricalcium phosphate was added two hours and three hoursafter completion of temperature rising, respectively. At these times,rates of polymerization were 40% and 49%, respectively.

Then, when the content of the vessel was kept at 90° C. for 2.5 hoursand a polymerization rate of 95% was achieved, 6 g of tricalciumphosphate and 0.3 g of sodium dodecylbenzenesulfonate were added again.The concentration of oxygen at this time was 0.4 vol %. Thereafter,substitution with nitrogen was stopped, the vessel was completelysealed, and 600 g of styrene was added dropwise continuously over 3hours while the temperature was raised to 100° C.

The procedures thereafter including impregnation with an expanding agentand surface-processing were repeated in the same way of Example 10.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 3.

Comparative Example 4

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 6,000 g of styrene, 20 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was heated to90° C., and a pipe for blowing was opened to allow polymerization toproceed. After elapses of 2 hours and 3 hours after completion oftemperature rising, 3 g of tricalcium phosphate was added, respectively.At these times, rates of polymerization were 34% and 43%, respectively.

Then, when the content of the vessel was kept at 90° C. for 3.0 hoursand a polymerization rate of 95% was achieved, 6 g of tricalciumphosphate and 0.3 g of sodium dodecylbenzenesulfonate were added again.When the concentration of oxygen in the polymerization vessel at thistime was measured, it was 13.0 vol %. Thereafter, the vessel was heatedto 100° C. over 1 hour.

The procedures thereafter including impregnation with an expanding agentwere repeated in the same way of Example 10.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 3.

TABLE 3 Ex. 13 Ex. 14 Ex. 15 Timing of Substitution of Inside ofReaction Vessel with Before Start Before Start Before Start Nitrogen ofof of Polymerization Polymerization Polymerization Concentration ofInitial Stage of Polymerization (vol %) 0.6 0.9 0.1 or Lower Oxygen inReaction Late Stage of Polymerization (vol %) 0.6 1.0 0.1 or LowerVessel Weight Average Whole Particle (A) 297,000 300,000 255,000Molecular Weight 1/5 Portion from Center 220,000 219,000 202,000 3/5Portion from Center 221,000 224,000 205,000 6/6 of 1/5 Portion fromSurface 370,000 380,000 438,000 5/6 of 1/5 Portion from Surface 375,000380,000 4/6 of 1/5 Portion from Surface 382,000 385,000 3/6 of 1/5Portion from Surface 384,000 395,000 2/6 of 1/5 Portion from Surface390,000 395,000 441,000 1/6 of 1/5 Portion from Surface (B) 398,000395,000 446,000 (B)/(A) × 100% 134 132 175 Expandability (ml/g) 70 70 75Flexural Strength of Molded Article (MPa) 0.312 0.308 0.305 Ex. 16 Comp.Ex. 1 Comp. Ex. 4 Timing of Substitution of Inside of Reaction Vesselwith Before Start — No Nitrogen of Polymerization Concentration ofInitial Stage of Polymerization (vol %) 0.7 — 20.8 Oxygen in ReactionLate Stage of Polymerization (vol %) 0.4 — 13.0 Vessel Weight AverageWhole Particle (A) 300,000 301,000 295,000 Molecular Weight 1/5 Portionfrom Center 216,000 232,000 295,000 3/5 Portion from Center 216,000265,000 293,000 6/6 of 1/5 Portion from Surface 382,000 336,000 297,0005/6 of 1/5 Portion from Surface 384,000 338,000 4/6 of 1/5 Portion fromSurface 387,000 339,000 3/6 of 1/5 Portion from Surface 394,000 340,0002/6 of 1/5 Portion from Surface 396,000 348,000 299,000 1/6 of 1/5Portion from Surface (B) 405,000 332,000 295,000 (B)/(A) × 100% 135 110100 Expandability (ml/g) 70 70 67 Flexural Strength of Molded Article(MPa) 0.310 0.295 0.285<Examples of Keeping Concentration of Oxygen at 7 Vol % or Lower FromStart to Late Stage of Polymerization Without Adding AdditionalMonomers>

Example 17

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 6,000 g of styrene, 20.8 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the polymerization vessel was sealed, anda pipe for blowing was opened to feed nitrogen into the vessel. When theconcentration of oxygen became 6.5 vol %, the pipe for blowing wasclosed. After the vessel was heated to 90° C., 3 g of tricalciumphosphate was added two hours and three hours after completion oftemperature rising, respectively. At these times, rates ofpolymerization were 34% and 43%, respectively.

Then, when the content of the vessel was kept at 90° C. for 3 hours, 6 gof tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonate wereadded again. When the concentration of oxygen in the polymerizationvessel at this time was measured, it was 5.8 vol %. Thereafter, thevessel was heated to 100° C. over 1 hour.

Then, 90 g of cyclohexane was added under pressure, and after 1 hour,420 g of butane (isobutane/normal butane ratio=4/6) was added underpressure in 1 hour, and then the content of the vessel was kept foranother 4 hours. Thereafter, the content was cooled to room temperatureand then taken out of the autoclave.

After the taken out slurry was washed, dehydrated and dried, theresulting product was classified by passing through a 14 mesh and beingcaught in a 26 mesh and then coated with 0.08% of zinc stearate, 0.05%of castor hardened oil and 0.02% of dimethyl silicone to obtainexpandable styrene resin particles (average particle diameter: 0.85 mm).

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 4.

Example 18

Expandable styrene resin particles (average particle diameter: 0.85 mm)were obtained in the same manner as in Example 1 except that nitrogenwas fed into a polymerization vessel continuously between completion ofcharging and completion of polymerization at 90° C. to control theconcentration of oxygen in the polymerization vessel to 0.1 vol % orlower.

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 4.

Comparative Example 5

In a 14-liter autoclave equipped with an agitator, 6,000 g of purewater, 9 g of tricalcium phosphate, and 0.3 g of sodiumdodecylbenzenesulfonate were charged under an agitation of 230 rpm. Ahydrogen ion concentration at this time was 8.0.

Subsequently, 6,000 g of styrene, 20.8 g (Wet 75%) of benzoyl peroxide,2.4 g of t-butyl peroxyisopropylcarbonate, and 3 g of ethylenebisamidewere charged under agitation.

After completion of charging, the vessel was heated to 90° C., and apipe for blowing was opened to allow polymerization to proceed. Theconcentration of oxygen in the polymerization vessel at this time was20.7 vol %. After elapses of 2 hours and 3 hours after completion oftemperature rising, 3 g of tricalcium phosphate was added, respectively.At these times, rates of polymerization were 33% and 42%, respectively.

Then, when the content of the vessel was kept at 90° C. for 3 hours, 6 gof tricalcium phosphate and 0.3 g of sodium dodecylbenzenesulfonate wereadded again. When the concentration of oxygen in the polymerizationvessel at this time was measured, it was 15.8 vol %. Thereafter, thevessel was heated to 100° C. over 1 hour. The procedures thereafterincluding impregnation with an expanding agent were repeated in the sameway of Example 1 to obtain expandable styrene resin particles (averageparticle diameter: 0.85 mm).

The molecular weight and properties of the obtained expandable styreneresin particles were measured, and the results are shown in Table 4.

The invention is based on Japanese Patent Applications Nos.P2002-132623, P2002-132624, P2002-242015, P2002-242016, P2002-280359 andP2002-381173, which are herein incorporated by reference.

It is to be understood by those skilled in the art that the forgoingdescription relates to preferred embodiments of the invention and thatvarious changes and modifications may be made in the invention withoutdeparting from the spirit and scope of the appended claims. Also it isto be understood that the invention is not limited to the embodimentsthereof except as defined in the appended claims.

TABLE 4 Comp. Ex. 17 Ex. 18 Ex. 5 Conditions for Concentration of Oxygenin Reaction Vessel 6.5 0.1 20.7 Polymerization (After Charging) (vol %)Concentration of Oxygen in Reaction Vessel (at 5.8 0.1 15.8 Completionof Polymerization) (vol %) Results of Weight Average Molecular Weight ofOutermost 301,000 306,000 295,000 Evaluation Layer Portion (OneExpandable cell) Weight Average Molecular Weight of Surface 298,000299,000 299,000 Layer Portion (Two Expandable cells) Appearance 93 96 85(Degree of Smoothness of Surface) (%) Expandability (ml/g) 68 68 68

1. A process of producing expandable styrene resin particles,comprising; suspension-polymerizing styrene monomers by using seedscomprising regenerated styrene resin particles and adding styrenemonomers, keeping the concentration of oxygen in a reaction vessel at 7vol % or lower when a polymerization rate is 60% or higher, andimpregnating styrene resin particles with an expanding agent before orafter completion of the polymerization.
 2. An expandable styrene resinparticle wherein a material of a cell of an outermost layer portionhaving a thickness corresponding to the size of one expandable cell hasa weight average molecular weight higher than a weight average molecularweight of a material of cells of a surface layer portion having athickness corresponding to the size of two expandable cells.